All aircraft have systems for measuring the pressure at the altitude of flight. Usually these are connected to openings called static ports, which are placed to minimize distortions caused by airflow around the airframe. Calibration of these systems is very important for operation of the aircraft because the altitude at which an airplane flies is determined from this pressure measurement. An incorrect pressure measurement may lead to an incorrect altitude and possible flight hazards. For this reason, aircraft manufacturers calibrate pressure-sensing systems accurately, often using a device called a trailing-cone system that trails a pressure sensing port behind the aircraft. Such systems are expensive to install, and their deployment in flight involves special precautions, however.
In order to safely fly an aircraft, a pilot must rely on accurate measurements of temperature, pressure, airspeed, humidity, and altitude. Accurate measurements are also very important in making scientific measurements via research aircraft. Many of the core measurements made from aircraft instruments are interconnected, however. For example, in measuring temperature, corrections may be made for dynamic heating caused by the motion of the aircraft. To measure airspeed, measurements of dynamic pressure, ambient pressure and temperature may be needed. Corrections may be further made to a measured pressure based on airspeed and/or orientation of the aircraft. Precisely measuring airspeed may depend on accurate knowledge of humidity, gas constants, and specific heats. Measurements of humidity by dew-point sensors may require correction for differences between ambient and sensor pressures. Because there are seldom standards or reliable references for any of these measurements, an analysis of uncertainty involves complicated and multi-dimensional examinations of measurement interactions and how flight conditions may influence measurements from otherwise carefully calibrated sensors.
It would be highly desirable to obtain a reliable reference in flight for any of the interlinked measurements to reduce measurement uncertainty. Trailing cones may be used to provide reliable measurements of pressure as aircraft airspeed, altitude and attitude angles change throughout a normal flight envelope. Disadvantages to a trailing cone include a need for a special and difficult installation, which may be particularly problematic for a pressurized aircraft flying at low pressure. Trailing cones are also not suitable for routine measurement.
Other prior methods include comparisons between aircraft. Research aircraft are often flown in formation to collect measurements, and if there are differences between those measurements it may be possible to determine which measurement is at fault. This method does not provide a reference measurement, however.
Other prior calibration methods include flight past towers with tethered balloon sensors, which may provide limited checks on the accuracy of measured pressures. These measurements are only possible at low altitude and low airspeed, however, and are not generally suited to the calibration of measurements in flight.
Other prior methods include ways to correct errors in measurements. For example, calibration has been possible via Global Positioning System (GPS) when the wind is known accurately by independent measurement. The aircraft drift measured with GPS may be compared to the expected drift, and the associated dynamic pressure may be corrected. Multiple measurements of pressure at ports around a spherical surface have also been used to determine the error in measured ambient pressure.
None of the prior methods provide a reliable airspeed measurement that may be used continuously and with a high degree of precision to correct ambient pressure, however. Nor do any of the previous methods provide a measure of temperature that is independent of the measure read directly from a temperature sensor. In addition, none of the prior airspeed measurement methods are valid both in clear air and during cloud penetrations. The present Application overcomes these and other problems and an advance in the art is achieved.